Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMAGING METHOD, APPARATUS, AND PRODUCT
This invention relates to an improved method and
apparatus for imaging a substrate having a coating thereon
which is transformable by means of a relatively low current
electrical discharge, and the product produced thereby.
More particularly, this invention relates to a method and
apparatus whereby commonly available diazo-type lithographic
printing plates may be electronically imaged inexpensively
and with relat;vely high resolution, and subsequently used
in conventional lithographic printing processes without
requiring a photocomposition or photo-typesetting step.
BACKGROUND OF THE INVENTION
In most lithographic printing systems in use today,
the lithographic printing plate is imaged by means of a
photographic process during which a
photographically-generated film positive or negative
transparency carrying the desired image is first prepared
and then projected onto or exposed in contact with the light
sensitive surface of the plate. In certain systems the
plate may be exposed directly by the original copy without
the need for an intermediate film transparency (i.e., by
reflection), but such systems still require the initial
preparation of "camera-ready" copy.
Attempts have been made to eliminate the need for art
and copy preparation, as well as the need for various
photoconversion process steps, by generating an image
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carrier, i.e., an imaged lithographic printing plate,
directly from electronically stored or generated data. Such
systems may, for example, rely upon a laser beam which
impinges upon a light sensitive plate surface, or rely upon
an electrical spark or arc, or other source of energy, which
removes one or more layers of material from the surface of a
lithographic-type plate, often a plate having a special
construction, or may use electrostatic charges to define the
desired image.
Lithographic plate imaging systems of these types
frequently have significant shortcomings, among the most
significant being one or more of the following: the
relative complexity and therefore high cost and low
reliability of the apparatus necessary to implement these
systems, the high cost of the specially formulated and
prepared unimaged lithographic plates which generally must
be used in such systems, or the generally low quality of the
resulting printed image generated by plates which have been
imaged by such systems.
Other known processes invented by others include a
process which provides an efficient, inexpensive system for
generating, for example, an image of high quality on a
variety of relatively inexpensive diazonium resin
lithographic plates of conventional design, without the need
for specialized plate coatings, or the need for
photocomposition, "camera-ready" art or copy preparation, or
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photoconversion steps, and preferably using image data which
is electronically generated or stored. In this previously
known process, which is the subject of Canadian Patent
Application Serial No. 456,702 of F.S.Love, a
relatively low current electrical discharge is used to
produce a latent image capable of conventional image
development on the plate surface by inducing a chemical
change in the material found on the face of the plate (which
may be a photopolymer or other plate, although diazonium
resin-type plates are generally preferred) which changes the
relative solubility of the plate coating in the areas traced
by the discharge, without displacing or removing significant
quantities of the coating material as is usually done wlth
spark-type systems, and without relying upon photo-;nduced
processes commonly encountered in laser systems.
A limitation of this previously known process is an
inability to generat~ line segments or dots of good print
quality having a width or diameter smaller than about 4 to 5
mils. In other words, by using the process disclosed by
Love, one is limited to generating images of good visual
quality having an effective print resolution of
approximately 200 lines per inch. The process is capable of
generating images having higher resolution, up to about 300
to 400 lines per inch, but only with a dramatic decrease in
image quality, i.e., irregular line widths and edges,
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unintended breaks in line segments, missing image dots or
groups of dots, etc. The imaging speed (i.e., speed at
which the stylus sweeps over plate areas to be imaged) is
relatively slow in the process disclosed by Love and would
be a substantial limitation in many commercial situations.
The method and apparatus of the invention disclosed
herein overcomes the limitation on effective print
resolution, and at the same time increases the imaging
speed. In but one of several embodiments, this invention
comprises contacting an unimaged printing plate with a low
current, localized type electrical d1scharge which is
aligned within a thin stream of a relatively inert gas, whch
is directed at the plate surface with a velocity of at least
about Mach 0.05.
Utilizing the teachings of this invention, the full
scope of which will be better understood upon reading the
description herein below, latent images on many diazonium
resin or other plates may be generated which contain line
segments or dots having a width or diameter of approximately
1 mil, which therefore makes posslble lithographically
printed images having an effective print gauge of
approximately 1000 lines per inch. It has been found that
the method and apparatus of this invention result in a low
current electrical discharge which is much more localized
than that of the process disclosed by Love. Because of this
increased localization, it is believed the resulting
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electrical discharge is significantly more intense with
respect to the area of discharge contact on the plate
surface. It is therefore possible to increase the speed at
which the discharge is made to sweep over the plate surface
with no significant degradation in image quality. It is
believed the relative linear speed of the discharge over the
plate surface can be increased by a factor of ten or more
when compared with the relative linear speeds possible using
the teachings of Love.
A detailed description of the invention follows, in
which yet other features and advantages of this invention
will become apparent, and in which reference is made to the
Figures summarized below.
Figure 1 schematically depicts an apparatus which may
be used to image a printing plate, in accordance with the
teachings of this invention.
Figure 2 is a section view of the apparatus shown in
Figure 1, taken along the lines II-II, showing details of
the stylus assembly;
Figure 3 is an enlarged section view of the tip of the
stylus depicted in Figure 2;
Figure 4 is a section view of the stylus assembly of
Figure 2, taken generally along the lines IV-IV;
Figure 5A is a diagrammatic enlarged section view of
the tip of the stylus of Figure 3, showing the "diffuse"
electrical discharge effect described herein;
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Figure 5B is a diagrammatic section view of the tip of
the stylus of Figure 3, showing the localized, non-pulsating
electrical discharge effect described herein;
Figures 6A through 6D are generalized plots showing
the parameter regions in which the various discharge effects
are observed;
Figure 7 is a schematic diagram of a switching circuit
which may be used in connection with this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
An apparatus and process embodying this invention as
applied to plates commonly used in conventional offset
lithography are schematically depicted in Figure 1. Roll 10
serves as a support for conventional lithographic plate 20,
Appropriate securing means 12, 14 may be employed to attach
plate 20 securely to roll 10 during the imaging process
described below. Means 12, 14 may be any means capable of
attaching and holding a lithographic plate of conventional
design on a roll surface, such as, for example, means
employing an array of tapered pins as is used by many offset
lithographic press manufacturers to attach and secure
lithographic plates having a row of holes for accommodating
the pins along each end. It is preferred that the plate
itself be at least moderately electrically conductive, so
that the plate may be electrically grounded during the
imaging process. To facilitate the grounding arrangement,
means 12, 14 may be designed to afford d grounding path from
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plate 20, as, for example, by electrically grounding roll 10
and ensuring that means 12, 14 are in electrical contact
with both the roll 10 and the conductive plate 20 Other
arrangements for electrically grounding plate 20 may be used
as well.
To facilitate placing the desired latent image on the
roll, roll 10 may be rotated by means of motor 5 and belt
drive 7 Angular displacement sensor 16 may also be
associated with roll 10. Such sensor ~ay be used to
indicate the precise rotational position of roll 10, and is
particularly desirable if the pattern is to be placed on the
plate automatically by electronic means, as is discussed
below.
Opposite the surface of roll 10 and plate 20 is
positioned an electrical imaging assembly, shown generally
at 30. In the embodiment depicted in Figures 1-4, assembly
30 comprises an electrode or stylus 31 for establishing an
electrical discharge within a discharge region between
electrode or stylus 31 and the opposing surface of plate 20,
gas supply line 48 for supplying the discharge region with a
gas in the manner disclosed herein, and ionization promotion
means 55 directed into the discharge region between the tip
of stylus 31 and the opposing surface of plate 20. A
primary function of electrical imaging assembly 30 is to
establish and interrupt, or modulate, in accordance with
externally supplied pattern information, a particular kind
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of localized electrical discharge within the region between
the surface of plate 20 and the tip of stylus 31 which is
capable of forming a latent image on the surface of plate 20
in accordance with the teachings herein. Imaging assembly
S 30 is described in more detail below.
Stylus 31 is a hollow needle-l;ke shaft having an
internal bore diameter of between about 0.001 and about
0.008 inch, through which an inert gas may be directed at
velocities greater than about Mach 0.05. Stylus tip radii
within the range of about .002 inch to about .004 inch have
been used with success, although tip radii outside this
range may be advantageous in certain applications. Stylus
31 may be constructed of any suitable electrically
conductive material.
As depicted in Figures 2 through 4, stylus 31 may be
constructed from a disposable steel needle 32 of the kind
normally associated with a hypodermic syringe, which has
been modified by securely fitting a section of tubing of the
appropriate dimensions within the distal end of needle 32.
Such attachment may be made by adhesive bonding, soldering,
brazing, welding, etc. If necessary, tubing of the
appropriate inside diameter may be made by crimping tubing
having a larger than desired inside diameter about a solid
wire having the desired diameter, and retracting the solid
wire. Stylus 31 may be partially encased in a block of
protective, electrically insulating material, depicted at 35
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in Figures 2 and 4, to electrically isolate and facilitate
handling and positioning of the stylus tip. Block 35 in
turn is secured within stylus assembly block 40.
As depicted in Figures 2 and 4, block 40 has a channel
44 of circular cross section which extends through the
central portion of the block. This channel is sized so as
to snugly accommodate collar 42 associated with needle 32.
If desired, block 40 may be made in the form of two or more
mating sections of acrylic plastic or other suitable
material. Screw 38 may be used to provide electrical
connection, via tab 39, between stylus 31 and an appropriate
electrical source, discussed in more detail below. As is
apparent from the Figures, one end of channel 44 houses
stylus 31 and its associated fittings. The opposite end of
channel 44 is securely connected, by means of fitting 46, to
a source, not shown, of an ionizable gas which is at some
pressure above atmospheric and which is relatively inert
with respect to the plate surface in the absence of an
electric discharge. Among others, commercially available
relatively inert gases intended for spark chamber
applications have been found generally suitable and
satisfactory. Such gases can have helium and neon as
principal constituents. The presence of oxygen in the
discharge gap appears to inhibit the imaging process of this
invention. ~enerally speaking, a relatively uniform gas
pressure sufficient to generate a stream of ionizable gas
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through stylus 31 at a velocity within the range of about
Mach 0.05 to about Mach 0.~ or higher is desired; it is
contemplated that stream velocities of up to about Mach 0.9,
or perhaps even higher, may be used.
The tip of stylus 31 is preferably positioned radially
perpendicular to the surface of plate 20, at a distance
ranging from about 0.001 inch to about 0.010 inch, via
micrometer stage 50, although it should be emphasized that
distances outside this range are known to be operable.
Hereinafter, the immediate region between the tip of stylus
31 and the opposed surface of plate 20 shall be referred to
as the discharge gap. To facilitate scanning of the stylus
across the face of plate 20, the stylus may be attached to a
translating stage 52 capable of precisely controllable
motion along the rotational axis of roll 10; data specifying
the relative position of stage 52 is preferably made
available to a pattern data processor 75 to facilitate the
relative positioning of stylus 31 over the surface of plate
20 and to assure proper synchronization of the flow of
pattern data to the stylus to maintain accurate image
re-creat;on on the plate surface.
Ionization or breakdown promotion means 55 may be any
means which is found effective in promoting electrical
breakdown of the gas in the discharge gap. It is believed
that use of such means 55 can minimize certain time lags
associated with electrical discharges in gases by acting as
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a source or generator of free electrons or negative ions
which initiate the subsequent secondary ionization processes
ultimately responsible for the avalanche behavior leading to
breakdown. By use of such means 55, the time lag between
the application of the requisite electrical potential
between the stylus and the plate and the establishment of
the particular kind of electrical discharge utilized in this
invention may be reduced dramatically. It should be noted
that the voltage necessary to initiate such breakdown is
lowered by the action of means 55 as well. In one
embodiment of the invention, means 55 may be a shielded
corotron device, comprising a short section of tungsten wire
positioned within a semi-cylindrical, electrically grounded
shield, similar to that commonly employed in electrostatic
copying machines as an ion source for charging the
xerographic plate. Alternatively, a relatively 10w-powered
ultraviolet light source directed into the discharge region
may be used, as is depicted at 55 in Figures 2 and 4.
Optionally, no ionization or breakdown promotion means need
be employed.
It is believed that the electrical discharge
phenomenon utilized in this invention is separate and
distinct from the arc or spark discharge phenomenon
described generally in the literature. Much of the
literature addresses electrical discharge phenomena which
occur at low or extremely low pressures, and wherein a
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relatively large anode is positioned at à substantial
distance from a relatively large cathode. In the instant
invention, however, the electrical discharge may occur at or
near atmospheric pressure, and occurs between a needle-like
stylus and a substantially flat plate, with a gap spacing of
only perhaps 0.001 to 0.010 inch or so. Typical
time-averaged electrical current values may range from about
2xlO 6 amperes to abou~ 2xlO 3 amperes, although operation
above this range, but below the spark discharge regime, may
be preferred under some conditions. With commonly available
diazonium resin plates, time averaged current values within
the range of from about 2xlO 5 amperes to about 2xlO 3
amperes have been found to be preferable. It is therefore
not possible to directly correlate some of the physical
parameter values recited herein with all references found in
the literature. It appears clear, however, that electrical
arcing, as the term is generally understood in the
electrical discharge art, is not involved. Arcs may be
generally categorized as high current electrical discharges,
20 involving currents greater than one ampere or so, rather
than the relatively low time-averaged currents discussed
herein. (See e.g., Gaseous Electronics, Volume I, Edited by
Hirsh and Oskam, pages 294-295)
It is also believed that the electrical discharge
phenomenon utilized in this invention is distinct from the
electrical discharge phenomena described by Love. By using
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the voltages, gap sizes, and gas-introduccion techniques
taught by Love, the resulting electrical discharge creates a
bright glow region within the discharge gap which, when
viewed with the aid of a microscope, has a generally
outwardly flaring cross section ~measured perpendicular to
the axis of the stylus~, as depicted in Figure 5A. The glow
region generally contacts the plate surface in an area which
is substantially larger than the cross-sectional area of the
stylus, and appears somewhat diffuse. Furthermore, the
voltage measured between point P of Figure 1 and ground with
the aid of an oscilloscope during such discharge may be seen
to have a small oscillating component which causes the
observed waveform to have a distinct sawtooth appearance,
with the sawtooth component having a frequency within the
range of about 20 kHz to about 200 kHz, and an amplitude on
the order of 10 volts or less. This type discharge may
therefore be described as visually diffuse and electrically
pulsating.
It has now been discovered that the introduction of a
gas in accordance with the teachings herein, e.y., via the
bore of perpendicularly-oriented hollow stylus 31, when
employed with the other process parameters disclosed herein,
can cause the electrical discharge utilized by Love to
undergo a change in which the width or cross-sectional area
of the observed glow region within the discharge gap, when
viewed with the aid of a microscope, appears to shrink both
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abruptly and dramatically, resulting in a much more
localized electrical discharge which appears to emanate
principally from within the distal end or bore of stylus 31
and which has a distinct, bright non-flaring "core" as
depicted in Figure 5B. The resulting glow region appears to
maintain a generally uniform diameter as it progresses
toward the plate surface; the area of the plate surface
which appears to be contacted by the electrical discharge is
th~refore approximately equal to the area of the bore of
stylus 31. Additionally, the voltage measured between point
P (Flgure 1) and ground with the aid of an oscilloscope
during such discharge contains no observable pulsations or
sawtooth-like components. This localized discharge is
significantly different than the diffuse discharge observed
using the teachings of Love even with coaxial feeding of the
gas, as depicted in Figure 5A, wherein the discharge flares
outwardly from the stylus tip to the plate surface, and
wherein the central bright "core" flares outwardly as well,
resulting in a discharge "footprint" on the plate which is
substantially larger than the area of the bore of stylus 31.
This difference is believed to result in an ability to image
a plate using a localized-type discharge, as described
above, with a resolution totally beyond that available using
the teachings of Love, as well as an ability to image a
plate using much higher relative tracing speeds between
stylus and plate surface (i.e., significantly less
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"exposure" time to the influence of the discharge is
required to bring about the changes in the plate surface
necessary to generate a latent image) than is possible with
the diffuse-type discharge techniques taught by Love.
Figures 6A through 6D diagrammatically illustrate the
different discharge ef~ects which have heen observed with
the apparatus depicted in Figure 1. These graphs represent
generalizations of experimental plots of applied voltage,
measured from point P (Figure 1) and ground, versus gap
size, obtained with a hollow stylus having an inside
diameter of 0.002 inch and using helium gas at the indicated
flow rate. As may be seen, for voltage levels below a
certain minimum threshold, no discharge of any kind is
observed regardless of gap size (Region I). Increasing the
voltage results in the establishment of a diffuse-type
discharge, and increasing gap size generally dictates
increasing the gap voltage to maintain the discharge (Region
II). Increasing the voltage above a second threshold value
while maintaining a smàll gap spacing results in the abrupt
transition of the discharge from the diffuse type to the
localized type (Region III).
The relative voltage values "A" and "B" indicated in
Figure 6A represent operating points which may be used to
take advantage of the speed at which this abrupt transition
takes place in controlling the pattern or image imparted to
the plate. First a roll speed is chosen which is slow
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enough to permit a localized type discharge to induce the
necessary insolubilizing effect on the plate, yet fast
enough to render the diffuse type d;scharge totally
ineffective in imaging the plate. One may then set the
relative voltage levels so that a non-imaging diffuse type
discharge is present at all times (i.e., at voltage level
"B") except when an image is to be formed, at which time a
voltage pulse at voltage level "A" is used to form an image
by initiating an abrupt transition from the diffuse type
discharge to the localized type discharge.
It is bel;eved that introduction of the thin gas
stream ;nto the discharge region and against the plate
surface area to be imaged need not be coaxial with the
stylus or electrode, although this is a preferred
embodiment. ~ithin broad limits, the electrical discharge
appears to follow preferentially the flow of gas, even if
the path of the gas stream is not along the line defining
the shortest distance between the stylus tip and the plate.
It is believed this is due to the fact that the ionization
potential within the gas stream is significantly lower than
the ionization potential in the ambient air surrounding the
gas stream, thereby making ionization within the gas stream
much more likely. It is therefore foreseen that, under
certain conditions, the stylus and the means by which the
thin gas stre~m is introduced and directed onto the plate
may be separate structures mounted independently in
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proximity to the discharge gap, and need not be mounted
perpendicularly to the surface of the plate. Of course, if
high resolution is desired, it is important that the
cross-sectional area of the gas stream, as it contacts the
S plate, be minimized. It is therefore generally preferred
that the means for delivering the gas to the discharge
region be capable of forming the gas into a fine stream
having an appropriately small cross-sectional area, and
having the requisite velocity, i.e., a velocity above about
Mach 0.05, and preferably subsonic. In general, stylus bore
inside diameters within the range of about 0.002 to about
0.004 inch are preferred, which in turn generate
correspondingly thin gas streams; bore diameters outside
this range may be used so long as gas velocities of at least
about Mach 0.05 are generated.
It is believed the electrical discharge utilized in
this invention is not sufficiently intense to remove
significant or substantial quantities of the plate coating.
No physical change in the underlying plate surface is
observed. The nature of the transformation mechanisms
involved are not known. To what extent the same chemical
reactions which occur in conventional imaging (e.g.,
photographic) processes occur during the imaging process of
this invention is not known; it merely appears that the
post-treatment behavior of the resulting latent image is
generally similar~ although not always identical, to that of
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a conventionally imaged plate. It is believed the
electrical discharge forms a stream of ions which are
directed into the plate coating, and that the fast moving
stream of gas serves to "contain'l or "direct" the ;ons in a
stream which is more concentrated or focused, or less
diffuse, than the ion stream generated using the teachings
of Love. The interaction of these ions with the chemical
compunds in the coating is believed to cause a chemical
transformation in the coating which modifies the relative
solubility of the coating. The term "insolubilizing effect"
is used herein to mean the chemical (or whatever other)
effects which such discharge treatment has on these plates
which permit such plates to be developed and used in a
manner similar to conventional plates which have been
exposed or imaged by conventional (e.g., photographic)
methods.
Current limiting resistor 60 may be used to prevent
the electrical current between the stylus 31 and the surface
of plate 20 from becoming excessive. Excessive current can
result in the transition of the discharge phenomenon between
stylus 31 and plate 20 from the normal low current
electrical discharge used in this invention to the arc
discharge behavior described in the literature. Excessive
current can also result in the undesirable rearrangement or
removal of substantial portions of the coating on the
surface of plate 20. The use of a current limiting resistor
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prevents the discharge current from approaching values whichwould characterize an arc-type discharge. Because
increasing the time-averaged current within the discharge
gap also tends to increase the width of the image lines (see
Example IV), changing the value of such resistor allows for
adjustment of the width of the lines appearing on the
resulting imaged plate. Line width may also be adjusted by
varying the cross-sectional area of the gas stream, the
speed of the plate surface relative to the stylus, etc., if
desired.
The blocks designated 70, 75, 80, and 85 collectively
comprise electronic circuitry which expedite the generation
of a latent image on the surface of plate 20 in a timely
manner from electronically stored, generated, or transmitted
pattern data. Block 80 schematically depicts a
voltage source adapted to provide voltages within the range
of about 200 to about 2000 volts, at current levels within a
range of between about 2x10 6 and about 2x10 3 amperes.
Block 85 depicts a high speed switch capable of switching
the voltage generated by the voltage source 80 at the
frequencies necessary for the desired image resolution or
print gauge. The necessary switching frequency is, of
course, a function of the speed at which the surface of
plate 20 is traced over or scanned by stylus 31, the desired
image resolution, as well as other factors, such as the
delay between the application of the requisite voltage and
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the initiation of the desired electrical discharge. It has
been found that, for imaging small (e.g., 10 x 15 inch)
lithographic plates at a roll circumferential speed of about
72 inches per second and a print gauge of about 1,000 lines
per inch, switching frequencies within the range of about O
to about 72 kilohertz are necessary.
An example of one circuit which may be used in this
application is shown schematically in Fig. 7. The circuit
operates as follows: Data input at nodes A, B in the form
of a train of +5 volt pulses are shifted from a ground
reference to a bias voltage reference across nodes C, D by
voltaye level shifter 100. MOSFET Q2 then amplifies the
output of level shifter 100, relative to the bias voltage.
This results in a signal 1Lr GD wherein the +5 volt data
pulses are now on the order of +400 volts, referenced to the
~250 volt bias voltage. MOSFET Q2 acts as a voltage
follower, decoupling the output of MOSFET driver Ql from the
discharge gap and load resistor RL. Voltage lLr JF
represents the buffered, amplified, shifted data output of
high speed switch 85. The output voltage of approx1mately
650 volts is divided between current limiting resistor RL
and the gap formed between the stylus tip and the plate
surface.
The circuit depicted in Fig. 7 may be any by which a
logic signal of modest voltage (e.g., 5 volts) from the
pattern generation means may be impressed upon a relatively
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high voltage d.c. bias (e.g. 300 volts). For example, an
optical coupler such as that available from Texas
Instruments (of Dallas, Texas) as ~odel TIL 111 may be used.
Use of such a bias scheme permits the stage shown in Figure
7 to achieve switching of approximately 900 volts (to
ground) with only about 600 volts across the output
transistors.
The pattern data processor represented by block 75
represents the means by which the required switching
instructions dictated by the pattern data from block 70 are
sent to the high speed switch 85 so that the desired pattern
data is synchronized with the appropriate relative location
of the stylus on the face of plate 20, and in registry with
the latent image previously generated on the face of plate
20, thereby resulting in the proper switching of the
electrical discharge from the diffuse mode of Love to the
localized mode disclosed herein, and vice versa, as the
stylus sweeps over the areas of the plate intended to carry
the pattern. Any suitable means for generating or
retrieving such instructions may be employed. Pattern data
of course may be generated manually, but in most situations
electronic generation or retrieval of pattern data or
switching instructions is preferred, for example, through
the use of analog or digital data storage means such as
magnetic or paper tape, a ROM, RAM, or EPROM, a bubble
memory, etc. Appropriate data from the angular displacement
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sensor 16 associated with roll 10 may be input to processor
75 to facilitate the necessary task of converting the
pattern data to a series of switching instructions to
switching means 85, and translating instructions to
translating means 52 which will result in a train of
correctly timed and sequenced electrical discharges of the
desired type between stylus 31 and the relatively moving
face of plate 20.
It is contemplated that, in addition ~o merely
generating a series of "on-off" switching instructions which
would sequentially establish and extinguish the electrical
discharge in digital fashion in rigid accordance with
pattern data, one may wish to modwlate the electrical
discharge by varying the discharge current over a range of
values which lie substantially within the discharge current
envelope recited herein. The range of values may be defined
by a series of pre-determined discrete levels, or,
alternatively, may employ a more-or-less continuum of values
within pre-determined limits. In either case, the desired
result is the ability to vary the effective area in which
the latent image-forming chemical transformation takes place
within the surface of plate 20 traced by the stylus, and
thereby vary the effective resolution or effective print
gauge of the imaging process. Generally speaking, lower
time-averaged current values result in narrower lines formed
on the plate surface. By employing this method for
05~
discretely or continuously modulating the current associated
with establishing or maintaining a low current electrica'l
discharge, latent images containing well-formed lines or
dots having a wide range of widths or diameters are
possible. For example, dots of excellent quality and
uniformity having diameters of approximately one mil have
been produced on ordinary diazonium resin plates by using a
localized-mode discharge, as taught herein (see Example I).
Furthermore, one or more dots of almost any desired diameter
within the range of the system may be generated, without the
need for having to limit dot size to one of a relatively few
available choices, or having to build larger "dots" from the
mass1ng or aggregation of smaller dots of unifor~ size, as
is done in many conventional laser systems. Such capability
is advantageous in producing latent images wherein extremely
fine detail or half-tone graphics are desired.
While the plates suitable for use wlth this invention
are conventional, photosensitive lithographic plates which
may be imaged by conventional methods using a high intensity
light source, it is believed that the light produced by the
low energy electical discharge employed in this invention
does not contribute substantially to the imaging process.
The intensity of the light given off by the electrical
discharge employed herein is quite modest by conventional
plate exposure standards. The apparent diameter of the
visible electrical discharge is comparable to the resulting
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line width which is produced by the discharge. No maskingof nonimaged areas from the light gener~ted from the
discharge appears to be necessary. Sharp, microscopically
well^defined boundaries may be observed between those lines
tr~ced by the electrical discharge and areas immediately
adjacent to such lines, which areas have been exposed to the
light of the discharge, but not to the discharge itself. It
is believed the experiment described in Example V indicates
that the light of the discharge is not a principal or
primary contributor to the insolubilizing effect which the
discharge is observed to have on conventional diazonium
resin plates.
Following the operation of one embodiment of this
invention, pattern data information generated or stored in
data source 70 is fed to pattern data processor 75, which
receives the instructions along with data on the rotational
position of roll 10 from angular displacement sensor 16.
Processor 75 then generates two sets of instructions. One
set of instructions is sent to translating stage 52 to
assure correct placement of the imaging stylus along the
axis of roll 10. A second set of instructions is sent to
high speed switch 85 to generate the train of voltage pulses
necessary to establish the sequence of electrical discharges
which serve to image the plate with the desired pattern
information.
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1;~4~0~;3
The current sent from switch 85 passes through RL, a
load resistor which serves to limit the direct current
delivered to stylus 31, which stylus may be precisely spaced
from the surface of plate 20 by means of a micrometer
assembly S0 or other means. For most plates imaged using
the process disclosed herein, the voltage impressed upon
stylus 31 is electrically positive with respect to the
plate, although in some cases, a negative stylus polarity
may be preferred.
Imaging assembly 30 is traversed across the face of
plate 20 by translating stage 52. A controlled quantity of
a relatively inert gas such as helium or a mixture of helium
and nkon is directèd, via stylus bore 33, into the discharge
gap~ i.e., the region between stylus 31 and the surface of
plate 20, in the direction of plate 20 at a velocity in
excess of about Mach 0.05. Ionization promotion means 55 is
directed toward the region between stylus 31 and the surface
of plate 20 for reasons discussed above. Motor S is used to
turn roll 10 at a constant rate via belt 7 thereby allowing
stylus 31 to scan over the entire surface of plate 20, which
is temporarily but securely attached to the perimeter of
roll 10 and allowing the electrical discharge from stylus 31
to sweep over all pattern areas on the plate. By
establishing and controlling the requisite electrical
discharge in the pattern or image areas of, for example,
diazonium resin plates under these conditions, the plate
- 25-
i24105~
surface in those areas becomes resistant to (i.e.,
relatively insoluble in) the developing materials used in
developing such plates. By carefully adjusting the desired
voltage and gap size, and by properly establishing the flow
of gas, in accordance with the teachings herein, the
preferred localized electrical discharge of this invention
may be utilized to image plates with an image resolution of
approximately 1,000 lines per inch. These plates, imaged by
the electrical discharge process of this invention, may
thereafter be developed using conventional developing
techniques.
The electrical discharge employed in this invention
has been used to place a latent image exhibiting extremely
fine detail, as well as excellent solid image areas, on a
variety of commercially available, lithographic-type
printing plates under a variety of operating conditions, as
may be determined from the following illustrative examples,
which are not intended to be limiting in any way.
Additive-type plates have been found to be particularly
suitable for use in connection with this invention.
EXAMPLE 1
An apparatus similar to that schematically depicted in
Figure 1 was used, in accordance with the following
specifications and operational parameter values:
Plate: 3M "R", a negative working, additive-type
photosensitive plate distributed by 3M
- 26-
~2~105~
Corporation, St. Paul, Minnesota, conventionally
mounted with the metallic plate surface
electrically grounded to the electrically
conductive roll via conventional conductive
fastening pins attaching the plate to the roll.
Stylus: Hollow steel tube having an inside
diameter of 0.002 inch, and a tip radius of
approximately 0.002 inch. The stylus tip is
spaced approximately 0.0015 inch from the plate
surface. The stylus shank is embedded in the
bore of a hypodermic needl~. A channel is
provided for the delivery of gas to the discharge
area via the bore of the tube, as depicted in
Figures 2 and 4.
Gas: Helium, fed through the configuration
indicated in Figures 2 and 4 at a flow rate of 31
standard cubic centimeters/minute.
Breakdown Promotion Means: A UV lamp Model
22SC-35 (Ultra Violet Products, Inc. San Gabriel,
California 91778) is positioned in close
proximity (approximately 0.75 inch) to the stylus
tip and plate surface (see Figures 2 and 4).
- 27-
~ ~,4~053
Current Limiting Resistor (RL): 5.0U Megohms,
0.5 watt.
High Speed Switch: Similar to that depicted in
Figure 7.
Source of Pattern Data: PDP 11/40 Computer
(distributed by Digital Equipment Corporation,
Maynard, Massachusetts) with appropriate
associated electronics of conventional design.
With the above-specified apparatus, the plate was
continuously rotated on the roll with a roll circumferential
speed of approximately 72 inches/second. Ambient light in
the vicinity of the apparatus was subdued to prevent fogging
of the photosensitive plate. Applied voltage of the
electrical discharge during the imaging period was 900
volts, resulting in a localized type discharge, and was
maintained at 300 volts during the non-imaging period,
resulting in a diffuse type discharge, with the polarity of
the voltage on the stylus being positive at all times with
respect to the grounded plate roll. The stylus WâS slowly
and automatically traversed along the axis of roll rotation
at a rate of approximately 0.2 inch/minute, thereby causing
the stylus to trace a closely spaced helical path on the
plate surface. The desired pattern was a typogrâphicâl test
- 28-
~24~0~3
form pattern, requiring minimum dot diameters of
approximately 0.001 inch. The maximum switching frequency
was about 72 kilohertz.
After the latent image of the desired pattern was
created on the coated surface of the plate by contact with
the localized type electrical discharge, the plate was
removed from the roll and developed conventionally, i.e.,
the plate was treated with process gum (R Process Gum* a
product of 3M Corporation of St. Paul, Minnesota), then with
lG a lacquer developer (Reliable Red Lacquer Developer,*
distributed by Anchor/Lithkemco of Hicksville, ~ew York) for
image reinforcement, and then treated wlth a 50-50 (by
weight) mixture of gum aràbic and distilled water. Dur;ng
the development process, the areas contacted by the
localized type electrical discharge appeared to behave
similarly to areas on similar plates imaged conventionally,
i.e., imaged photographically via actinic light, while areas
contacted by the diffuse type discharge behaved as though
unexposed. The resulting developed plate exhibited visually
outstanding detail. The plate was then placed on a sheet
fed offset lithographic duplicator of conventional design.
The ink chosen was 0/S Washfast Black NC 23424 distributed
by Sinclair and Valentine of Charlotte, North Carolina. The
dampening solution was 3M Duplicator Fountain Concentrate,
distributed by 3M Corporation of St. Paul, Minnesota,
diluted as dirested (1-15 parts by volume). The paper
*Trademark - 29-
i2~ ;3
chosen was a conventional white business paper having a
basis weight of 20 pounds, distributed by International
Paper Company. The resulting printed sheets carried an
exceptionally clear and detailed image corresponding to the
areas of the plate contacted by the localized type
electrical discharge, with no undesired background.
EXAMPLE II
The procedures of Example I were followed, except that
the plate used WaS an LKK*brush grain "wipe-on" type plate,
of 6 mil thickness, and LKK 6110 Sensitizer Base and LKK
6110 Sensitizer Powder was used, all distributed by
Anchor/Lithkemco of Lynbrook, New York, and the load
resistor used was 2.5 Megohms. As in Example I, the image
obtained on the plate was well defined, with excellent
contrast and resolution.
-
EXAMPLE III
The procedure of Example I was followed except that
the plate used was a 3M type "E" plate, distributed by 3MCorporation, St. Paul, Minnesota. The plate was imaged
using the invention disclosed herein, and was developed as
in Example I. The image obtained on the plate was well
defined, with excellent contrast and resolution.
*Trademark
- 30-
~ .Q 5
EXAMPLE IV
The plate of Example I was imaged and developed in
accordance with the teachings of Example I, except that the
gas flow rate was 25 standard cubic centimeters/minute, roll
S circumferential speed was 20 yards/minute, and the
resistance values for the current limiting resistor RL were
varied over a range which permitted monitoring of current in
the discharge region. The approximate associated line width
achieved (based upon reproduction of a series of spaced
parallel lines) is given below.
- 31-
124~053
Line
RL (Megohms) Current (Microamps) ~idth (mils)
9 64.5 2.2
7 83.9 2.4
116.1 2.7
3 200 3.0
2 296.7 3.4
1 580.5 4.1
Image quality in all cases was excellent.
EXAMPLE Y
A small section of Mylar-brand polyester plastic film
having a thickness of 5 mils and having a conductive coating
on one side was interposed between the stylus and the
surface of the plate of Example I, with the conductive
coating facing the stylus. The plastic film is distributed
by Sierracin Corporation of Sylmar, California under the
trademark Intrex~. The plastic film as used was partially
transparent to visible light, the transmission having been
measured over the visible region of the spectrum and
averaging approximately 50%. The conditions of Example I
were used to cause an electrical discharge to be established
between the stylus and the conductive surface of the film,
except that the gas flow rate was 25 standard cubic
centimeters/minute, the relative speed between the stylus
and the film surface was 0.1 inches/second, and the stylus
- 32-
~2~1~153
was positioned approximately 0.002 inch from the film
surface. To take into account the partial transmissivity of
the film to light and the increase distance between the
stylus and the plate, the rate of traverse of the stylus
across the surface was made smaller than in Example I. The
conductive surface of the film was suitably grounded to the
roll carrying the plate.
It was observed that the characteristics of the
various discharges between the stylus and the conductive
film surface, in particular, the shape and luminosity of the
discharges, were substantially identical to those which were
observed in Example I. After exposure as above, the film
was removed from the swrface of the plate and the plate was
developed as in Example I. No developed images of any kind
were observed.
It is believed that visible light alone, as produced
by the discharge of the instant invention, is insufficient
in intensity to form a latent image on this plate. Because
the light intensity to which the plate is exposed in this
Example is calculated to be approximately ten ti~es the
intensity to which the plate is exposed in Example I (having
taken into account the transmissivity of the film and the
increased spacing of the discharge from the plate surface
because of the finite thickness of the film), it is
therefore concluded that, in the practice of this invention
as exemplified in Examples I through IV above, the
- 33-
~4~05~3
impingment of ions on the surface is the principal
mechanism, and that the incidential production of light
associated with the electrical discharges of this invention
does not play a significant role.
To insure that light from a conventional source, if
sufficiently intense, would be capable of producing a latent
image after passing through the partially transmissive film
used above, a plate identical to the one used above was
partially covered with a smal1 section of film as used above
and then exposed to a source of conventional light. After
exposure, the film section was removed and the plate
developed as abové. It was observed that both the regions
directly exposed to the light and those which were beneath
the film developed to the same degree, thus establishing the
fact that the film is capable of transmitting at least some
of those wave lengths of light to which the plate surface is
sensitive.
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